The ethanol stream (broth) coming from the fermentor in a fermentation process for producing biofuel (ethanol) contains a significant amount of water and some unfermented solids. To recover fuel grade ethanol in this stream involves a water removal step (dehydrate), wherein water typically accounts for >85 wt % of the fermentation broth. The conventional process of removing water from the ethanol stream is via distillation, and the overhead stream from the rectifying column of the distillation step is sent to a molecular sieve for further dehydration to approach pure ethanol. The use of distillation for alcohol recovery is energy intensive because the heat supplied is also used to vaporize the water, thus reducing thermal efficiency. In addition, there is a limit to the degree of ethanol purity that can be achieved with conventional distillation. For example, distillation is a poor choice for separation once the ethanol-water mixture reaches the azeotropic composition (96.4 wt % ethanol). Other distillation methods, such as azeotropic distillation and extractive distillation, are applicable but they are all energy intensive processes and in most cases involve introducing additional separation steps to the process for ancillary solvent recovery which add cost to the process.
One common feature of any fermentation technology pathway is the need for ethanol recovery post-fermentation and this is currently being done using distillation. Based on ethanol production energy analysis, ethanol recovery step is reported to account for more than 45% of the total energy requirement for a given plant. It is therefore highly desirable to find alternative technologies/processes that are less energy intensive than a distillation process.
Various approaches have been or are currently being explored to find a solution to the problem of reducing the energy consumption of the ethanol recovery step in ethanol production plants. One of such approaches is the development of membrane-based processes for ethanol dehydration, e.g. pervaporation (PV) and vapor permeation (VP). Pervaporation is a separation process in which a liquid mixture is brought into contact with a semi-permeable membrane on the feed side, and the membrane selectively removes one component (mainly due to stronger affinity) to the permeate side while rejecting the other components with lower affinity to the residue or reject side. Vapor permeation is similar to pervaporation, however the feed is in vapor phase. Membrane processes are reported to consume less energy than distillation and may offer energy savings of roughly 50% depending on the membrane material's productivity and separation efficiency. Membranes also require less plant footprint than distillation processes. Membrane operations have the potential to be simpler in comparison to distillation operations. An issue with membrane processes without distillation is the need for solids removal prior to the dehydration of ethanol and pretreatment of the broth solids as contaminants can impact the performance of the membrane unit. Patent application 2007/0031954 describes an integrated process which includes multiple membranes which include a first membrane separation, followed by dephlegmation step, and another membrane separation for ethanol dehydration. A problem with this approach is the need for solids removal from the fermentation broth before the membrane separation process. The solids removal step creates additional capital investments and operating costs which increases the overall ethanol production cost.
Mechanical vapor recompression (MVR) is an approach that has not been used for ethanol dehydration MVR concepts have been reportedly used in the form of single effect evaporation for desalination of water, for concentrating black liquor in the paper industry, and for wastewater treatment. MVR is reportedly less energy intensive than distillation, and therefore an integrated process comprising a MVR unit operation and a membrane separation process should offer significant cost savings in ethanol production. In a MVR process, the vapor generated from a column or evaporator type arrangement is recompressed, to elevate the pressure and temperature, and then heat exchanged with the feed to the column or evaporator. The vapor generated in the MVR unit should be free of solids and non-volatile contaminants for ethanol—water separation. The ethanol concentration in the vapor from the MVR needs to be treated further to meet desired specifications.